An example of a scattering medium which is of interest for photoluminescence imaging (in short luminescence imaging) or photoluminescence tomography (in short luminescence tomography) is biological tissue. Tissue optics is a field devoted to study the interaction of light with such tissue. Over the last decades, the field has grown rapidly. With increasing knowledge of the light-tissue interaction, the interest in applying tissue optics as a diagnostic tool is also emerging, reaping the fruits from the fundamental research.
An area in tissue optics, which the present disclosure is partly dealing with, is photoluminescence imaging including photoluminescence tomography, which are non-invasive approaches for in-vivo imaging of humans or animals. These imaging approaches are luminescence-based and require an external source of light for excitation of luminescent biological markers.
Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. A specific form of luminescence is fluorescence, where typically emitted photons are of lower energy than those used for illumination. Thus, in fluorescence, the fluorescent wavelength is Stokes shifted to a longer wavelength with reference to the wavelength of the illuminating light.
Fluorescent imaging is known and can, for example, be used to study biological responses from drugs in small animals over a period of time, without the need to sacrifice them.
Shimomura, Chalfie and Tsien were rewarded with the Nobel prize in 2008 for discovering and developing the green fluorescent protein, which has become a very important fluorescent marker.
However, hitherto, fluorescence molecular imaging and tomography systems for diffuse luminescent imaging or diffuse luminescent tomography in absorbing and scattering media suffer from a number of drawbacks. They have for instance a low resolution or contrast, which makes diagnostic tasks based on the imaging results difficult. Hence, there is a need for such systems having improved image quality, e.g. by improved contrast and/or resolution of the two-dimensional or three-dimensional images provided.
Further, these systems are sensitive to ever-present endogenous tissue autofluorescence, deteriorating measurement results. Since the fluorescence signal from the fluorescent biological markers and the background autofluorescence often overlaps, separating them is difficult and often not reliably possible.
The autofluorescence conceals the fluorescence signal when using Stokes-shifted fluorophores, effectively limiting the signal-to-background sensitivity.
Thus, there is a need for an improved diffuse luminescent imaging or luminescent tomography system, method or luminescent markers for luminescent imaging or luminescent tomography which in particular allow for increased effectiveness by improved contrast and/or improved imaging resolution.